Many semiconductor manufacturing operations are performed at low pressures and high temperatures. Processing modules are often kept at low pressures while wafers are transferred between low and high pressure environments using load locks. Load locks effectively isolate two environments and eliminate the need for repeatedly cycling processing modules, which typically have large internal volumes, between different pressure levels. Instead, only small volume load locks are cycled during wafer transfers. Some configurations include several processing modules integrated with one or more internal wafer handling modules on the low pressure side of the processing system. Wafers may go through several processes without being transferred to a high pressure environment.
After processing, wafers must be removed from a low pressure processing module and placed into an atmospheric environment for, e.g., storage. Such wafers may need to be cooled to certain temperatures before being exposed to oxygen to prevent oxidation, out-gassing, and damage to storage modules. Rapid but uniform cooling is highly desired but is often difficult to achieve. To maintain high throughput, only a few seconds are afforded for the entire transfer process. Wafers often need to be cooled by more than 200° C. usually by positioning wafers close to a cold surface, e.g. a cooling pedestal. Often wafers are not flat and require relatively large set distances between the cold surface and a default wafer position to avoid direct contact. Since it is not practical to adjust the distances individually for each wafer, the separation distance must be set to non-optimum value (corresponding to the worst possible wafer deformation), resulting in generally poor heat transfer. Thus, load locks employ expensive venting gases such as helium, which has a high heat transfer coefficient, and even then prolonged cooling periods are required. A non-uniform gap between the cooling surface and the wafer also causes uneven cooling, leaving a hot spot in the areas bowing up (away) from the cooling surface, which could cause excessive stress possibly leading to wafer breakage.
Some solutions include electrostatic or vacuum clamping mechanisms to modify the shape of heat deformed wafers. Unfortunately, these solutions require large contact areas with wafer backsides, thereby increasing the risk of damaging the wafers and uneven cooling at contact points. Furthermore, the required clamping mechanisms are complex and expensive.
Therefore, there is a need for improved methods and apparatus that provide effective cooling during wafer transfer in load locks.